1. Field of the Invention
The subject invention is directed to cardiac leads, and more particularly, to a high impedance drug eluting bipolar cardiac pacing lead.
2. Background of the Related Art
Implantable cardiac stimulation leads, including endocardial leads, are well known in the art. In general, these devices have an elongated flexible body with an electrode at one end for contacting cardiac tissue and a connector at the other end for mating with an automated stimulation device, namely a pacemaker. The electrode of an endocardial lead may be secured within a chamber of the heart by passive fixation through the use of a plurality of flexible tines which project outwardly from the end of the lead body, or by active fixation through the use of a helical fixation screw.
When an endocardial lead has been implanted in the heart, either by active or passive fixation, it has been determined that the cardiac tissue at the site of implantation will react favorably to the lead in the presence of a therapeutic drug, such as, for example, a steroid. Consequently, cardiac leads have been designed with means for delivering a therapeutic drug to the cardiac tissue at the implantation site.
One such example of a lead having drug delivery means is disclosed in U.S. Pat. No. 5,902,330 to Ollivier et al. which describes a pacing lead having a frusto-conical diffusion ring fixed in place by gluing. The diffusion ring is constructed of porous silicone and loaded with an active material, such as a steroid, for distributing the steroid in the region of the myocardium adjacent the electrode. Another example of a drug delivery device is disclosed in U.S. Pat. No. 6,361,780 to Ley et al. which describes a microporous bio-compatible collar or annulus having a therapeutic drug within its pores. The collar or annulus is preferably formed from a ceramic material and is designed to surround a portion of a lead or catheter.
In cardiac stimulation leads, it is advantageous to employ a high impedance electrode at the point of contact with the cardiac tissue. This provides more efficient stimulation of the heart tissue with lower current drain from the power source. This reduction in energy utilization, results in extending the battery life of the pacemaker.
One way to achieve high impedance is to increase the resistance of the conductors. However, this wastes energy. Another way to achieve high impedance is to minimize the geometric surface area of the stimulation electrode. Such an approach is disclosed, for example, in U.S. Pat. No. 5,282,844 to Stokes et al., the disclosure of which is herein incorporated by reference in its entirety Unfortunately, as the surface area of the pacing electrode decreases, it is more likely to cause trauma by penetrating into the cardiac tissue. Consequently, there are limitations as to size of the electrode.
It would be beneficial to provide a high impedance cardiac lead that overcomes the deficiencies of the prior art. More particularly, it would be beneficial to provide a high impedance cardiac lead that has a relatively small atraumatic pacing surface.
The subject invention is directed to a new and useful high impedance, bipolar cardiac lead. The lead includes an elongated flexible body formed from a bio-compatible insulative material and having opposed proximal and distal ends. An anodic ring electrode is operatively associated with the distal end of the lead body, and a cathodic tip electrode is coaxially disposed within the ring electrode so as to form an annular gap therebetween. An insulating member is disposed within the annular gap between the tip electrode and the ring electrode. A conventional lead connector is operatively associated with the proximal end of the lead body and is electrically connected to the ring and tip electrodes. Conductor means extend through the lead body to deliver energy from the connector to the ring and tip electrodes to stimulate cardiac tissue.
Preferably, the cathodic tip electrode has an exposed surface with an atraumatic convex configuration that essentially takes the form of a segment of a sphere. To achieve the requisite high impedance, the atraumatic cathodic tip electrode has an exposed surface area in the range of about between 0.50 mm2 and 0.70 mm2. In accordance with a preferred embodiment of the subject invention, the insulating member disposed between the anodic ring electrode and the cathodic tip electrode is adapted to elute an anti-inflammatory drug once implanted in a chamber of the heart. Preferably, the insulating member is formed from a compound that includes silicone rubber and a steroid.
The lead is preferably adapted for passive fixation and thus includes a plurality of flexible tines associated with a distal end portion of the lead body. It is envisioned however, that the lead of the subject invention may include an active fixation screw. The conductor means preferably includes a multifilar conductor coil having a number of filaments associated with the ring electrode and a number of filaments associated with the tip electrode, and the connector is preferably defined by a bipolar IS-1 type connector.
These and other aspects of the high impedance, drug eluting, bipolar cardiac lead of the subject invention will become more readily apparent to those having ordinary skill in the art from the following detailed description of the invention taken in conjunction with the drawings described hereinbelow.